Last week we got to the point where we had a guitar with one pickup and no controls. This week we’ll be taking more steps towards a full understanding of guitar wiring.
Cut That Out!
Assuming we don’t want to stop there, the simplest next step would be to add a ‘kill switch.’ This is a switch that silences the guitar in one position and allows the signal through in the other. You might think that we can simply add a mini-toggle switch in the hot wire to cut the output from the pickup, like so:
However, when we use this switch to cut the signal, this scheme would actually cause the same noise we hear when we have a cable plugged into the amp but no guitar. We’re not ensuring that the two contacts are at equal voltage.
Instead, we need a switch that still completes the circuit, but cuts the pickup out of the equation, like this:
With this switch, in the ‘on’ position, the hot jack contact is connected to the output of the pickup. In the ‘off’ position, it’s connected straight to ground (while the hot output from the pickup isn’t connected to anything at all). Now we have a kill switch that truly silences the guitar when engaged.
Turn It Up
A kill switch is OK, but even more useful is a volume control. A volume control uses a potentiometer, which is the component that lives behind the knobs on a guitar. This is what it looks like:
As you can see, there are three contacts on the pot. The outer two are connected to either end of a resistive strip, and the middle one is connected to a ‘wiper’ that moves across the strip as the knob is turned. By connecting our hot signal to the leftmost contact, and the rightmost contact to ground, we can give the middle contact a controllable ‘choice’ between being connected completely to the hot output, completely to ground, or anywhere in between. By connecting this middle contact to the jack, as in this diagram, we have implemented a volume control.
In this diagram you can see that I’ve moved the ground connection from the pickup to the back of the volume control, and connected the ground connection to the same place as well as the volume’s third lug. It’s standard in guitar wiring that the ground wires are always connected to ground, for simplicity’s sake and to ensure that metal parts are grounded. It’s also fairly standard that the back of the volume pot is used as a grounding point for all wires to be grounded. There are pros, cons and exceptions, but a discussion of those is beyond the scope of this post.
Tone It Down
The last thing we’re going to look at in this article is adding a tone control. A tone control works differently to a volume control. It uses a pot and a capacitor together to ‘bleed’ the treble frequencies in the signal to ground. This works because putting a capacitor on a hot wire only allows treble frequencies to the other side. Once those treble frequencies are present on both hot and ground, the voltage difference is eliminated and the treble disappears from the audible output.
To attach a tone control to the circuit, we connect the input to the volume control (our hot signal from the pickup) to a second pot, at one end of the resistive strip. Then we connect a capacitor between the wiper contact of the pot and ground (for which we’ll use the pot casing). The other contact on the pot is unused, because we’re using it as a variable resistor in this instance, rather than as a voltage divider. Turning the pot down allows more signal to reach the capacitor, where the treble frequencies then leak through and are grounded. This is how it looks:
That’s the last thing I’m going to explain in this post. We now have a guitar circuit with one pickup, and a master volume and tone control. That’s the exact circuit used in the prototype Fender Esquire.
In the next post in this series we’ll go into multiple pickups and switching. If you have any questions so far then please do post them in the comments, and I’ll do my best to answer them as quickly as I can.